Abstract
Most SAGD projects require about one to two years for ramp-up. Over this period
of time, oil rate will be below peak oil rate and SOR will be higher than
long-term steady-state SOR. This paper discusses the effect of steam injection
pressure on SAGD ramp-up time, the associated geomechanical effects and
optimization of the ramp-up phase of SAGD. Different steam injection pressures
induce different reservoir geomechanical behaviour in oil sands. Higher steam
injection pressure is capable of inducing more favourable reservoir
geomechanical effects (such as shear dilation and isotropic unloading),
improving the reservoir permeability, and subsequently, benefiting the
long-term SAGD operation. This paper indicates that the ramp-up time can be
reduced due to the favourable geomechanical effects. A coupled reservoir
geomechanical simulation technique was applied for this investigation. In
addition, cap rock integrity concerns when applying high injection pressure are
also addressed. It is recommended that during or following the ramp-up phase,
the injection pressure be lowered to a safe operating pressure to ensure cap
rock integrity. The effects of low and high steam chamber pressures on SAGD oil
rate are also discussed.
Introduction
The steam-assisted gravity drainage (SAGD) process has been proven to be the
most promising technology for developing the Athabasca oil sands reserves in
northern Alberta. In 2001, only four commercial SAGD projects were operating in
the field(1). By March 2006, the number of active SAGD projects
increasedto 15(2).
Field SAGD operation experiences and numerical simulation show that the steam
injection pressure plays an important role in SAGD production performance. In
general, higher steam injection pressure helps lift fluid from downhole to the
surface, increases the oil production rate, reduces the overall well life and
improves the ultimate oil recovery. Some of these enhancements could result
from geomechanical effects induced by higher steam chamber pressure. For the
unconsolidated oil sands reservoir under certain in situ stress conditions,
higher steam injection pressure tends to induce larger volumetric strain
associated with shear dilation, thermal expansion, and even, tensile failure.
As a result, reservoir permeability can be improved and oil recovery will
beaccelerated.
Higher steam injection pressure, however, can also have undesirable effects. It
may cause the reservoir cap rock to be breached due to geomechanical behaviour,
and then result in a very high steam-oil ratio (SOR). Therefore, the time for
applying high steam injection pressure needs to be optimized for maximum
geomechanical benefits without causing cap rock failure. From the beginning of
steam injection to the time when the partially drained zone approaches the cap
rock, higher injection pressure may be safely applied. This strategy will
accelerate the oil production rate for the ramp-up phase and will attain the
peak SAGD oil rate sooner than otherwise possible.
This paper applies a coupled reservoir geomechanical simulation (called
"coupled simulation") technique(3) to investigate the relationship
between the steam injection pressure and the SAGD ramp-up process.
© 2009. Petroleum Society of Canada (now Society of Petroleum Engineers)
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History
- Original manuscript received:
23 June 2006
- Meeting paper published:
13 June 2006
- Revised manuscript received:
17 November 2008
- Manuscript approved:
1 December 2008
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